Door Speakers 190E 1992

Is there any way to test whether the door speaker, Bass Boost Amp, located in the passenger side rear trunk is operating properly? I am getting no sound, only slight vibration. "battery test" makes popping
sound, ran wires from dash speakers, and found door speakers work replaced fader control, still nothing. Amp dealer price is 275.oo, but they say they've "never heard of one going bad". I never knew they were there until I removed door panel. Any thing else I should check? Thanks...........Rick

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